1. Field of the Invention
The present invention relates to an apparatus for reproducing signals from an optical disk, and more particularly to a high speed Viterbi detector based on radix-8 and radix-16 for reproducing signals from an optical disk by executing a new state metric finding operation for a plurality of states at one time over a period of a plurality of clocks. The present application is based on Korean Patent Application No. 2000-64521, which is incorporated herein by reference.
2. Description of the Related Art
An optical channel can be modeled with a simple Partial Response (PR), and the reproduction apparatus can perform the PR processing using a Viterbi detector. Partial Response signal processing is a method for reproducing a symbol by extracting information from several adjacent data symbols. In other words, while a data symbol passes a channel, it affects the several adjacent data symbols. Thus, this characteristic can be utilized in recovering the data symbol.
The characteristics of the optical channel depend on a density of data symbol, a gap between tracks, characteristics of an optical pickup and a modulation scheme, etc.
There have been proposed Viterbi detection method and apparatus for finding a simple transfer function having the maximum likelihood with the characteristics of an optical channel, and for use in a partial response maximum likelihood (PRML) system for reproducing signals from a high-speed optical medium.
The characteristics of a channel can be modeled into a form of a simple convolution encoder, and, for example, if a constraint length of the transfer function k=4 in the modeling, the number of states of the convolution encoder becomes 2kxe2x88x921=8. However, the data encoded by an Eight-to-Fourteen modulation (EFM) scheme are restricted to have a signal conversion interval more than 3T.
FIG. 1 is a trellis diagram of a conventional radix-2 Viterbi detector for use in reproducing data signals from the optical disk in a conventional optical disk system.
According to the characteristic of a run-length limited code, RLL(2, 10) code, used in the conventional optical disk system, signals having a period of 1T and 2T do not exist therein. Thus, the states xe2x80x98+1xe2x88x921+1xe2x80x99 and xe2x80x98xe2x88x921+1xe2x88x921xe2x80x99 are removed so that the trellis state comprises 6 states. Also, the trellis paths are simply formed not to produce 1T and 2T signals.
FIG. 2 is a block diagram of a radix-2 Viterbi detector for use in a conventional optical disk system for implementing the trellis shown in FIG. 1.
Referring to FIG. 2, the conventional Viterbi detector comprises a branch metric calculation unit (BMC) 2, an addition/comparison/selection unit 4 and a path memory 8.
At every clock signal, one data symbol is input to the conventional Viterbi detector. Then, for every input symbol, the branch metric calculation unit 2 calculates 5 branch metrics that correspond to the absolute values +MAX, +MID, ZERO, xe2x88x92MID, and xe2x88x92MAX of the 5 differences between the input data symbol level and the 5 reference voltage levels.
The addition/comparison/selection unit 4, then, adds the 5 branch metrics generated by the branch metric calculation unit 2 and previous state metrics stored in it, respectively, to obtain the new state metrics, and selects the minimum new state metric thereamong to determine a survival path. In this processing, the addition/comparison/selection unit 4 performs the overflow controls in order to prevent any overflow of the state metrics. Specifically, the addition/comparison/selection unit 4 monitors all the state metrics and if it detects that all the state metrics exceed a predetermined value it subtracts a certain value from all the state metrics.
The path memory 8 stores the selected survival paths sequentially output from the addition/comparison/selection unit 4 and outputs them serially.
All of the branch metric calculation unit 2, the addition/comparison/selection unit 4 and the path memory 8 constituting the conventional Viterbi detector operate at the same channel clock speed which is equal to the input speed of the data symbol.
As such, the branch metric calculation unit 2 performing the simple subtraction and the absolute value operation, and the path memory 8 just performing exchange of data do not give any significant overhead to increase the operational speed of the Viterbi detector.
However, the addition/comparison/selection unit 4 performs, over one clock period, the complicated operations comprising the steps of adding the branch metric and previous state metric according to the trellis structure, comparing the new state metrics to obtain the survival path, which corresponds to the minimum state metric, and operating to prevent overflow of each of the state metrics and then obtaining the final new state metric for use in the next clock cycle.
Therefore, the complicated operation of the addition/comparison/selection unit is considered as an obstruction for implementing a high speed Viterbi detector in the optical disk market which is intensified with competition for the higher operation speed.
Accordingly, an object of the present invention is to provide a high speed Viterbi detector including an addition/comparison/selection unit for operating at one-third of the clock speed by reconstructing the trellis structure based on a radix-8 structure.
Another object of the present invention is to provide a high speed Viterbi detector including an addition/comparison/selection unit for operating at a one-fourth of the clock speed by reconstructing the trellis structure based on a radix-16 structure.
In order to achieve the above-described objects of the present invention, there is provided a radix-8 Viterbi detector for use in a signal reproduction apparatus for a high speed optical disk, including a frequency dividing unit for dividing a main clock frequency to generate an auxiliary clock at one-third of the main clock frequency; a branch metric calculation unit for calculating each branch metrics via a predetermined transfer function using the input data at the main clock frequency; a serial-to-parallel converting unit for outputting each of the branch metrics calculated by said branch metric calculation unit at the main clock frequency in units of 3 state bits; an addition/comparison/selection unit for adding the branch metrics input from said serial-to-parallel converting unit and previous state metrics which are stored before the 3 state bits are input and for comparing the addition results to select and output the minimum of the addition results as a new state metric, and for outputting a corresponding path selection signal, at the auxiliary clock frequency; a path memory for storing the path selection signal and for outputting parallel data corresponding to said path selection signal, at the auxiliary clock frequency; and a parallel-to-serial converting unit for converting the parallel data output by said path memory into serial data.
In addition, there is provided a radix-16 Viterbi detector for use in a signal reproduction apparatus for a high speed optical disk according to a preferred embodiment of the present invention, including a frequency dividing unit for dividing a main clock frequency to generate an auxiliary clock at one-fourth of the main clock frequency; a branch metric calculation unit for calculating each branch metric via a predetermined transfer function using the input data at the main clock frequency; a serial-to-parallel converting unit for outputting each of the branch metrics calculated by said branch metric calculation unit at the main clock frequency in units of 4 state bits; an addition/comparison/selection unit for adding the branch metrics input from said serial-to-parallel converting unit and previous state metrics which are stored before the 4 state bits are input and for comparing the addition results to select and output the minimum of the addition results as a new state metric, and for outputting a corresponding path selection signal, at the auxiliary clock frequency; a path memory for storing the path selection signal and for outputting parallel data corresponding to said path selection signal, at the auxiliary clock frequency; and a parallel-to-serial converting unit for converting the parallel data output by said path memory into serial data.
There is also provided a radix-N Viterbi detector employed in a signal reproduction apparatus for a high speed optical disk according to a preferred embodiment of the present invention, including a frequency dividing unit for dividing a main clock frequency to generate an auxiliary clock at one-Nth of the main clock frequency; a branch metric calculation unit for calculating each branch metric via a predetermined transfer function using the input data at the main clock frequency; a serial-to-parallel converting unit for outputting each of the branch metrics calculated by said branch metric calculation unit at the main clock frequency in units of N state bits; an addition/comparison/selection unit for adding the branch metrics input from said serial-to-parallel converting unit and previous state metrics which are stored before the N state bits are input and for comparing the addition results to select and output the minimum of the addition results as a new state metric, and for outputting a corresponding path selection signal, at the auxiliary clock frequency; a path memory for storing the path selection signal and for outputting parallel data corresponding to said path selection signal, at the auxiliary clock frequency; and a parallel-to-serial converting unit for converting the parallel data output by said path memory into serial data.